Abstract

Relative integrated cross sections are measured for rotationally inelastic scattering of hexapole selected in the upper Λ-doublet level of the ground rotational state in collisions with He at a nominal energy of 514 cm−1. Application of a static electric fieldE in the scattering region, directed parallel or antiparallel to the relative velocity vector v, allows the state-selected NO molecule to be oriented with either the N end or the O end towards the incoming He atom. Laser-induced fluorescence detection of the final state of the NO molecule is used to determine the experimental steric asymmetry, which is equal to within a factor of (−1) to the molecular steric effect, The dependence of the integral inelastic cross section on the incoming Λ-doublet component is also observed as a function of the final rotational spin-orbit (Ω′), and Λ-doublet (ε′) state. The measured steric asymmetries are significantly larger than previously observed for NO-Ar scattering, supporting earlier proposals that the repulsive part of the interaction potential is responsible for the steric asymmetry. In contrast to the case of scattering with Ar, the steric asymmetry of NO-He collisions is not very sensitive to the value of Ω′. However, the Λ-doublet propensities are very different for and transitions. Spin-orbit manifold conserving collisions exhibit a propensity for parity conservation at low but spin-orbit manifold changing collisions do not show this propensity. In conjunction with the experiments, state-to-state cross sections for scattering of oriented molecules with He atoms are predicted from close-coupling calculations on restricted coupled-cluster methods including single, double, and noniterated triple excitations [J. Klos, G. Chalasinski, M. T. Berry, R. Bukowski, and S. M. Cybulski, J. Chem. Phys. 112, 2195 (2000)] and correlated electron-pair approximation [M. Yang and M. H. Alexander, J. Chem. Phys. 103, 6973 (1995)] potential energy surfaces. The calculated steric asymmetry of the inelastic cross sections at is in reasonable agreement with that derived from the present experimental measurements for both spin-manifold conserving and spin-manifold changing collisions, except that the overall sign of the effect is opposite. Additionally, calculated field-free integral cross sections for collisions at are compared to the experimental data of Joswig et al. [J. Chem. Phys. 85, 1904 (1986)]. Finally, the calculated differential cross section for collision energy is compared to experimental data of Westley et al. [J. Chem. Phys. 114, 2669 (2001)] for the spin-orbit conserving transition